﻿using System;
using System.IO;
using System.Text;

namespace YalvLib.Providers
{
    /// <summary>
    /// This class contains functions that help detecting the correct encoding of a file.
    /// </summary>
    public class DetectEncoding
    {
        /// <summary>
        /// Opens the specified stream for reading.
        /// </summary>
        /// <param name="stream">The stream to open.</param>
        /// <param name="defaultEncoding">The encoding to use if the encoding cannot be auto-detected.</param>
        /// <returns>Returns a StreamReader that reads from the stream. Use
        /// <see cref="StreamReader.CurrentEncoding"/> to get the encoding that was used.</returns>
        public static StreamReader OpenStream(Stream stream, Encoding defaultEncoding)
        {
            if (stream == null)
                throw new ArgumentNullException("stream");
            if (stream.Position != 0)
                throw new ArgumentException("stream is not positioned at beginning.", "stream");
            if (defaultEncoding == null)
                throw new ArgumentNullException("defaultEncoding");

            if (stream.Length >= 2)
            {
                // the autodetection of StreamReader is not capable of detecting the difference
                // between ISO-8859-1 and UTF-8 without BOM.
                int firstByte = stream.ReadByte();
                int secondByte = stream.ReadByte();
                switch ((firstByte << 8) | secondByte)
                {
                    case 0x0000: // either UTF-32 Big Endian or a binary file; use StreamReader
                    case 0xfffe: // Unicode BOM (UTF-16 LE or UTF-32 LE)
                    case 0xfeff: // UTF-16 BE BOM
                    case 0xefbb: // start of UTF-8 BOM
                        // StreamReader autodetection works
                        stream.Position = 0;
                        return new StreamReader(stream);
                    default:
                        return AutoDetect(stream, (byte) firstByte, (byte) secondByte, defaultEncoding);
                }
            }
            else
            {
                if (defaultEncoding != null)
                {
                    return new StreamReader(stream, defaultEncoding);
                }
                else
                {
                    return new StreamReader(stream);
                }
            }
        }

        private static StreamReader AutoDetect(Stream fs, byte firstByte, byte secondByte, Encoding defaultEncoding)
        {
            var max = (int) Math.Min(fs.Length, 500000); // look at max. 500 KB
            const int ASCII = 0;
            const int Error = 1;
            const int UTF8 = 2;
            const int UTF8Sequence = 3;
            int state = ASCII;
            int sequenceLength = 0;
            byte b;
            for (int i = 0; i < max; i++)
            {
                if (i == 0)
                {
                    b = firstByte;
                }
                else if (i == 1)
                {
                    b = secondByte;
                }
                else
                {
                    b = (byte) fs.ReadByte();
                }
                if (b < 0x80)
                {
                    // normal ASCII character
                    if (state == UTF8Sequence)
                    {
                        state = Error;
                        break;
                    }
                }
                else if (b < 0xc0)
                {
                    // 10xxxxxx : continues UTF8 byte sequence
                    if (state == UTF8Sequence)
                    {
                        --sequenceLength;
                        if (sequenceLength < 0)
                        {
                            state = Error;
                            break;
                        }
                        else if (sequenceLength == 0)
                        {
                            state = UTF8;
                        }
                    }
                    else
                    {
                        state = Error;
                        break;
                    }
                }
                else if (b >= 0xc2 && b < 0xf5)
                {
                    // beginning of byte sequence
                    if (state == UTF8 || state == ASCII)
                    {
                        state = UTF8Sequence;
                        if (b < 0xe0)
                        {
                            sequenceLength = 1; // one more byte following
                        }
                        else if (b < 0xf0)
                        {
                            sequenceLength = 2; // two more bytes following
                        }
                        else
                        {
                            sequenceLength = 3; // three more bytes following
                        }
                    }
                    else
                    {
                        state = Error;
                        break;
                    }
                }
                else
                {
                    // 0xc0, 0xc1, 0xf5 to 0xff are invalid in UTF-8 (see RFC 3629)
                    state = Error;
                    break;
                }
            }
            fs.Position = 0;
            switch (state)
            {
                case ASCII:
                case Error:
                    // when the file seems to be ASCII or non-UTF8,
                    // we read it using the user-specified encoding so it is saved again
                    // using that encoding.
                    if (IsUnicode(defaultEncoding))
                    {
                        // the file is not Unicode, so don't read it using Unicode even if the
                        // user has choosen Unicode as the default encoding.

                        // If we don't do this, SD will end up always adding a Byte Order Mark
                        // to ASCII files.
                        defaultEncoding = Encoding.Default; // use system encoding instead
                    }
                    return new StreamReader(fs, defaultEncoding);
                default:
                    return new StreamReader(fs);
            }
        }

        /// <summary>
        /// Gets if the given encoding is a Unicode encoding (UTF).
        /// </summary>
        /// <remarks>
        /// Returns true for UTF-7, UTF-8, UTF-16 LE, UTF-16 BE, UTF-32 LE and UTF-32 BE.
        /// Returns false for all other encodings.
        /// </remarks>
        public static bool IsUnicode(Encoding encoding)
        {
            if (encoding == null)
                throw new ArgumentNullException("encoding");
            switch (encoding.CodePage)
            {
                case 65000: // UTF-7
                case 65001: // UTF-8
                case 1200: // UTF-16 LE
                case 1201: // UTF-16 BE
                case 12000: // UTF-32 LE
                case 12001: // UTF-32 BE
                    return true;
                default:
                    return false;
            }
        }
    }
}